The present invention relates to a sensor assembly and a method for determining a spatial position of a first part relative to a second part. The method and assembly can advantageously be used in a contact probe head for a coordinate measuring machine in order to determine the relative position of the sensing element of the probe head relative to a probe head base.
U.S. Pat. No. 4,866,854 discloses a sensor assembly using two Hall elements in order to determine a relative position. The two Hall elements are arranged on opposite sides of a magnet, so that the North pole and the South pole of the magnet each point towards a Hall element. Each Hall element produces an output signal representing the distance between the Hall element and the side of the magnet that is opposite thereto. If the magnet moves closer to one of the Hall elements, the distance to the other Hall element increases. In this way a position change of the first part relative to the second part along a measurement axis, which corresponds to the connecting line between the two Hall elements, is detected twice. A common sensor signal, which is representative of the actual position of the magnet between the two Hall elements, is produced by generating the difference between the two output signals. In some exemplary embodiments, U.S. Pat. No. 4,866,854 also proposes a plurality of magnets and a plurality of Hall element pairs so that the actual spatial position of the first part relative to the second part can be determined along a plurality of measurement axes. For this a magnet is required for each Hall element pair.
Hall elements respond essentially to a change of the magnitude of the magnetic field strength. By forming the difference of the output signals of each two opposing Hall elements, a common sensor signal is produced that is approximately proportional to the position of the magnet between the two Hall elements. The known sensor assembly allegedly has good sensitivity, selectivity and resolution. It is disadvantageous that the known sensor assembly requires a plurality of magnets for position determination in a plurality of measurement axes, which magnets should be disposed as far apart from each other as possible in order to achieve good decoupling between the sensor pairs. Each sensor pair requires “its” magnet. The required distances make it difficult to implement the known sensor assembly for a plurality of measurement axes in a compact manner. Furthermore, the assembly and the installation of the known sensor assembly in a device, such as the probe head of a coordinate measurement machine, require many individual assembly steps.
Sensor assemblies of the above-mentioned type are particularly necessary in coordinate measurement machines. Coordinate measurement machines usually comprise a so-called probe head, which is attached to a frame. With the aid of the frame the probe head can be moved relative to a measurement object. The probe head comprises a movable sensing element, typically in the form of a stylus. The probe head is moved towards the measurement object until the free end of the stylus contacts a target measurement point. The stylus is deflected relative to the probe head as a result of the contacting. Spatial coordinates, which are representative of the sensed measurement point, are then determined from the position of the probe head relative to the measurement object and from the position of the stylus relative to the probe head. Geometric dimensions and/or the shape of the measurement object can be determined by sensing a plurality of measurement points and determining the corresponding spatial coordinates.
DE 103 48 887 A1 discloses a probe head for a coordinate measurement machine. Again, pairs of Hall elements are used to determine the position of the stylus relative to the base of the probe head. As with the arrangement from U.S. Pat. No. 4,866,854, a Hall element pair is required for each measurement axis. Again, the individual Hall element pairs and the associated magnet are disposed at different points of the probe head as far apart as possible.
DE 10 2004 011 728 A1 discloses another probe head for a coordinate measurement machine having Hall elements and magnets. A Hall element is provided for each measurement axis and is disposed between two opposed magnets. Two measurement axes of the sensor assembly are combined on a supporting plate. The third spatial direction must be detected with a Hall element and an associated magnet pair, which must be separately mounted in the probe head.
As an alternative to Hall elements it has been envisaged to use so-called magnetoresistive sensors. DE 37 08 105 A1 discloses such a probe head for a coordinate measurement machine, wherein a single magnet is disposed on the end of the stylus closer to the probe head base. A plurality of magnetoresistive sensors is distributed on a detector surface that is disposed opposite the magnet. The known sensor assembly is quite compact. The measurement accuracy is limited, however, in particular because the Earth's magnetic field can cause unknown disturbances.
The structure and the principle of operation of the magnetoresistive sensors are known from various documents, for example from DE 195 21 617 C1 or from numerous publications in journals or at conferences. The sensors are frequently based on the so-called anisotropic magnetoresistive effect (AMR effect) or on the giant magnetoresistive effect (GMR effect). The AMR effect alters the electrical resistance of a ferromagnetic material through which a current is flowing depending on the angle between the current density vector and the magnetic field vector. The GMR effect is a quantum mechanical effect, which likewise causes a change of the electrical resistance of a ferromagnetic structure. AMR and GMR sensors have replaced the previously used so-called field plates. Field plates are magnetically variable resistances, in which the change in resistance is based on a change in the path length of the electrons in the magnetic field as a result of the Lorentz force.
DE 37 15 698 A1 discloses a probe head having three field plates, which are disposed on an outer circular ring, which is concentric to an inner circular ring having magnetic antipoles. Said sensor assembly appears quite compact. However, the measurement accuracy might be very limited owing to the sensors used and owing to the influences of the Earth's magnetic field.